Production Practices and Quality Assessment of Food Crops. Vol. 1

species show a progressive decline in photosynthetic rate under elevated CO 2 (Sage
et al., 1989; Miglietti et al., 1996). Elevated CO 2 has been shown to reduce the extent
of O 3-induced visible damage in some species (Rudorff et al., 1996; Mulholand et
al., 1997), but does not necessarily prevent reduction in photosynthetic rate (Balaguer
et al., 1995). In order to explain the protective influence of elevated CO 2 several
mechanisms have been proposed: (a) partial stomatal closure may reduce the
flux of O 3 into leaves (Allen, 1990); (b) increases in Rubisco content and activity
under elevated CO 2 may compensate for O 3-induced damage (Reid et al., 1998);
and (c) improved assimilate may increase the production of antioxidants (Rao et
al., 1995).
16.3. Transgenic plants
Impact of Ozone on Crops 201
Manipulation of antioxidative capacity could be a valuable way of obtaining ozonetolerant
plants. Screening analysis following plant transformation has shown that
with overexpression of superoxide dismutase or glutathione reductase in important
agronomical species such as tobacco, tomato, and potato, the transformant is
more resistant to photo-oxidative stress (Rennember and Polle, 1994; Foyer et al.,
1995). The responses of transformants to O 3 are dependent upon the type of isoenzyme
and the exposure regimen. Tobacco plants overexpressing superoxide dismutase
and glutathione reductase do not show enhanced tolerance to ambient O 3 levels
(Pitcher et al., 1991), whereas tobacco overexpressing superoxide dismutase shows
less visible foliar injury upon exposure to O 3 (Van Camp et al., 1994). Unfortunately,
the ecological significance of ambient O 3 on these transformed enzymes has still
to be elucidated. The study of these transformed plants will increase our present
understanding of O 3 phytotoxicity and will help to obtain O 3-tolerant plants.
17. CONCLUSIONS
Tropospheric O 3 has a negative impact on growth, development, and productivity
of crops. Effects of O 3 have been observed in a wide range of physiological characteristics,
such as accelerated senescence, decreased photosynthetic assimilation,
decreased productivity, and reduced carbon allocation to roots. Different responses
to O 3 have been observed in related species and, hence, it is thought that the initial
mechanism of O 3-induced stress on crops is uniform. A better understanding of
the effect of O 3 and O 3-generated reactive oxygen species is necessary for an
insight into the impact of O 3 in crops. A great effort must be made in order to
decrease the concentrations of O 3 air pollution. In the absence of an effective
emission control of O 3 precursors, it is expected that ambient O 3 concentration
will increase in the future. The responses of crops to increased tropospheric O 3
level could play an important role in determining crop yielding. Given that O 3induced
stress imposes considerable constraints on crop yield production, there is
a need for continuous research in this area. More work is necessary to better our
understanding of signal pathway and gene expression implicated in O 3 responses.